Spatial and temporal effects of direct drilling on soil structure in the seedling environment

Soil & Tillage Research 71 (2003) 163–173

Spatial and temporal effects of direct drilling on soil structure in the seedling environment Lars J. Munkholm a,∗ , Per Schjønning a , Karl J. Rasmussen b , Kari Tanderup c a

Department of Agroecology, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark b Department of Agricultural Engineering, Danish Institute of Agricultural Sciences, Research Centre Bygholm, P.O. Box 536, DK-8700 Horsens, Denmark c Department of Medical Physics, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus C, Denmark Received 4 September 2002; received in revised form 8 January 2003; accepted 28 January 2003

Abstract Despite more than 30 years of research and practical experience the interest in shallow tillage and especially direct drilling has remained low in Scandinavia. Excessive compaction of the topsoil layer is one of the major problems encountered when adapting shallow tillage and direct drilling in particular. The purpose of this study was to evaluate temporal and spatial effects of two different direct drilling techniques on bulk density and penetration resistance in the near seed environment. A sandy loam growing small grain cereals was followed during the first 3 years after conversion from conventional tillage to direct drilling to reveal short-term changes in soil structure. A field experiment with four blocks was conducted in 1999–2001 where a conventional mouldboard ploughing–harrowing system (PL) was compared with direct drilling performed by either a chisel coulter drill (DD-C) or a single disc drill (DD-D). Effects on density and penetration resistance were measured in the field after first, second and third year of crop establishment (T1, T2 and T3). Bulk density was determined at 0–100 mm depth using a dual probe gamma-ray transmission system. Penetration resistance was recorded in the field at 0–150 mm depth. At T2 column samples (diameter: 180 mm, height: 200 mm) were taken with the seed row through the centre. Penetration resistance was determined in these samples in a 10 mm × 10 mm grid using a micropenetrometer (3 mm cone base diameter) at 0 to approximately 150 mm depth. Two samples from each treatment were analysed by a medical CT-scanner to determine spatial differences in bulk density. Irrespective of coulter type direct drilling gave a fast compaction of the arable layer below seeding depth when shifting from mouldboard ploughing to direct drilling. Soil strength was substantially higher already in the first year of direct drilling (i.e., maximum 0.4 and 1.2 MPa, for PL and DD-D/DD-C, respectively). Critical high penetration resistance (>2.0 MPa) and bulk density levels (>1.5 g cm−3 ) were reached at T2 and remained at the same level at T3. The DD-C direct drill produced a more favourable soil environment for crop establishment than the DD-D drill. A layer of approximately 40 mm loose granular soil above seeding depth and no indication of a direct compaction effect was found for the DD-C treatment. In contrast, the field as well as the laboratory results indicated a direct compacting effect for the DD-D drill. Despite the lack of direct compaction effect from the DD-C drill itself, evidence suggest that periodic non-inversion soil loosening of the lower part of the arable layer is needed on direct drilled sandy loam soil in a moist and cool climate. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Direct drilling; Sandy loam; Denmark; Soil compaction; Spatial effects; Temporal effects

∗ Corresponding author. Tel.: +45-8-999-1768; fax: +45-8-999-1719. E-mail address: [email protected] (L.J. Munkholm).

0167-1987/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-1987(03)00062-X

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1. Introduction The extension of shallow tillage and especially direct drilling has remained low in Scandinavia despite more than 30 years of research and practical experience. Ball et al. (1998) concluded in a review that direct drilling is best suited for semi-humid or drier regions and in areas prone to soil loss by water erosion. The latter process is a major concern in mid-continental North American conservation tillage systems (Carter, 1994). Precipitation patterns in northern Europe are quite different from those of the great plains in North America, and most water erosion in northern Europe takes place during winter where there can be extended periods of rather low-intensity rain. In contrast, many erosion events in other parts of the world are triggered by intense storms in the growing season. In such conditions, the tillage system may be decisive for whether rather severe erosion processes are initiated. Most erosion events in northern Europe are less directly damaging on the soil quality, which may be the main reason for the historically limited interest among farmers in reduced tillage systems. Further to that, major drawbacks associated with direct drilling in Scandinavia have been problems concerning residue management, infestation of grassy weeds and excessive compaction of the topsoil layer (Riley et al., 1994; Rasmussen, 1999). In recent years improved technology has diminished the problems encountered in the past concerning residue management and weed control. However, excessive compaction of the topsoil layer may still be a significant obstacle for broader acceptance of continuous shallow tillage. Excessive soil compaction immediately below tillage depth under shallow tillage has been reported for humid regions at the northern hemisphere (Rasmussen, 1984; Rydberg, 1987; Schjønning and Rasmussen, 1989, 2000; Carter, 1991; Ball et al., 1994). The problem is particularly of importance in Denmark where soils in general are prone to compaction (i.e., moist climate and sandy poorly structured soils) and display low structural resilience. That is, they have no self-mulching capability. In addition, the moist and cool climate necessitates a porous soil with good aeration. The densification of the topsoil derived from direct drilling may be counteracted after some time by an increased number of biopores (Ehlers and Claupein, 1994). Few studies have addressed

the rate of change in topsoil structure under direct drilling. A number of studies have indicated that the degree of compaction under shallow tillage depend on the implement used. Schjønning and Rasmussen (1989) found excessive compaction under shallow tillage with a rotovator but not with a tine harrow for a fine loam. Also some types of direct drills have been shown to induce compaction. Double and triple-disc coulters producing V-shaped furrows has been found to induce furrow compaction and poor aeration under wet conditions (Baker and Mai, 1982; Chaudhry and Baker, 1988; Iqbal et al., 1998). Also problems concerning toxic elements produced from residues tucked into the seed furrow have been shown to arise from the use of disc drills in wet soil (Baker et al., 1996). In contrast, a chisel coulter producing inverted T-shaped seed furrows yielded no evidence of soil compaction (Baker and Mai, 1982). In addition, an overall better performance of the chisel coulter has been found under wet (Chaudhry and Baker, 1988) as well as dry conditions (Choudhary and Baker, 1982). The objective of our study was to evaluate temporal and spatial effects of two different direct drilling techniques on bulk density and penetration resistance in the near seed environment. A sandy loam was followed during the first 3 years after conversion from conventional tillage to direct drilling to reveal short-term changes in soil structure. The performance of a chisel drill developed in this project was compared with a commercially available single-disc direct drill. The direct drilling treatments were related to results from conventional tillage with mouldboard ploughing and seedbed preparation by tine cultivation.

2. Materials and methods 2.1. Soil type and tillage experiment A field trial addressing technology for row growing of small grain cereals in ploughless soil tillage systems (Jørgensen and Rasmussen, 2000) was established in 1999 at Research Centre Bygholm, Denmark (55◦ 53 N, 9◦ 48 E). The trial was conducted on a sandy loam that is classified as a Glossic Phaeozem according to the FAO system of classification. In the region,

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Table 1 Basic soil characteristics for the soil under investigation (0–200 mm) (soil water retention data from a neighbouring profile with similar texture)a Soil parameter Organic matter (g/100 g) Clay (<2 ␮m) (g/100 g) Silt (2–20 ␮m) (g/100 g) Fine sand (20–200 ␮m) (g/100 g) Coarse sand (200–2000 ␮m) (g/100 g) Water content at −100 hPa (m3 /100 m3 )

3 11 12 37 37 27

a Soil texture was determined using a combined hydrometer and wet-sieving method (Gee and Bauder, 1986). A LECO carbon analyser coupled to an infrared CO2 detector determined total carbon.

the 30-year mean annual precipitation (1961–1990) is 722 mm and the mean annual temperature is 7.5 ◦ C. Prior to the initiation of the tillage experiment, the study site had been under long-term conventional tillage that included mouldboard ploughing to a depth of about 200 mm. The entire experimental field was last ploughed in September 1997 about 19 months prior to the establishment of the experiment. Small grain cereals had been grown in the years preceding the tillage trial. Basic soil characteristics of the 0–200 mm layer are given in Table 1. Spring barley (Hordeum vulgare L.) was grown in 1999 and 2000 and winter barley in 2001. Spring barley was established on 3 May 1999 and 26 April 2000. Winter barley was established on 18 September 2000. The tillage treatments were applied to 6 m × 20 m plots in a randomised block design with four replicates. Direct drilling systems using either a single-disc coulter (DD-D) or a chisel coulter (DD-C) (Fig. 1) were applied and referenced by a traditional ploughing–harrowing tillage system (PL) (Jørgensen and Rasmussen, 2000). The disc coulter drill consisted of flat slightly angled wavy Väderstad coulters. The chisel coulter drill was developed in the project and consisted of a vertical disc followed by a 30 mm × 70 mm hoe tine with a 70 mm tall blade placed at the centre of the share (Jørgensen and Rasmussen, 2000). The row distance was 120 and 240 mm for the PL and direct drilling treatments, respectively. A wider row distance for the direct drilling treatments was chosen to allow inter-row hoe weeding. For the direct drilling treatments, drilling

Fig. 1. The direct drills used in the experiment: (A) single disc drill (DD-D) mounted with Väderstad coulters; (B) chisel drill (DD-C) consisting of a vertical disc followed by a 30 mm ×70 mm hoe tine with a 70 mm tall blade placed at the centre of the share.

was performed in the inter-row areas of the preceding crop. 2.2. Soil sampling and preparation Field tests and soil sampling were carried out in May 1999 (T1) and 2000 (T2) shortly after the emergence of the spring barley crops and in October 2000 (T3) shortly after the emergence of the winter barley crop. Undisturbed soil columns in plastic cylinders (180 mm diameter, 200 mm height) were collected from 0 to 200 mm depth in May 2000 (T2). One column was collected in each plot (i.e., four per treatment). All soil columns were sealed with plastic caps and stored at 2 ◦ C until analysis took place. In 2001 bulk soil samples from each block were taken for soil texture determination.

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2.3. Bulk density and water content Wet bulk density (WBD) was measured in situ with a high resolution dual probe gamma-ray transmission system (Nucletronics ApS, Magleby, Denmark) as described by Culley and McGovern (1990). The system consisted of two stainless steel probes placed 250 mm apart. One of the tubes held a 111 MBq (137 Cs) source and the other a scintillation detector with Li6 crystal. A high spatial resolution was achieved by only counting photons in a narrow range (window) of energy (Henshall and Campbell, 1983). The gamma photons were emitted from the source with an energy level of 0.662 MeV. A window of 0.635–0.880 MeV was chosen and a counting time of 60 s was selected. This enabled the system to integrate over a depth interval of only approximately 10 mm. WBD was calculated from the counts per second (cps) according to a laboratory calibration performed prior to the field measurements: WBD = −1.4096 log(cps) + 2.7688

(1)

Measurements were performed across and between rows at 0–100 mm depth for each 10 mm increment. The ‘across row’ measurements were performed in a band ±50 mm across the row. That is, the dual probe system was inserted in an oblique angle as related to the row. At T1 two series of measurements were made in each plot within two of the four experimental blocks (i.e., four series per treatment). At T2 and T3 one series of between and across row measurements (2–10 mm depth) were carried out in each plot, i.e., four replicates per treatment. Water content was determined employing a neutron method and time domain reflectometry (TDR). The neutron source (241 AM/Be, 1.11 GBq) and detector were located on the same probe rod as the gamma-ray detector for the dual probe gamma-ray system. Water content was determined by the neutron method at the same depths as for the WBD measurements (for each 10 mm increment at the 20–100 mm depth), applying a calibration performed immediately prior to the measurements. It was anticipated that biased results would be found for the very topsoil due to escape of neutrons to the atmosphere (Gardner, 1986). Initial test showed that this was the case at approximately 0–60 mm depth. At T2, water content was also determined using the TDR method at the 20–100 mm depth for each 20 mm increment. These data showed that,

although biased, no substantial errors in the estimates of dry bulk density was introduced by using the water contents derived from the neutron method also in the top 60 mm soil. Hence, dry bulk density was calculated from a combination of WBD and the volumetric water content determined by the neutron method for all soil layers. One series of between and across row measurements (20–100 mm depth) were carried out in each plot, i.e., four replicates per treatment. 2.4. Penetration resistance Penetration resistance was measured in the field to a depth of 150 mm with an automated cone penetrometer (Olsen, 1988). All measurements employed a 20.27 mm diameter cone with a 30◦ semi-angle (T1) or a 60◦ semi-angle (T2 and T3). Measurements were performed at a strain rate of 5 mm s−1 and with recording of penetration resistance for each 10 mm increment. The measurements were carried out within and between rows. The water content at these field tests is reported in Table 2. At T1 10 determinations within and between rows were made in each plot within two of the four experimental blocks (i.e., 20 replicates per treatment). At T2 and T3 five determinations were done within and between rows in each plot (i.e., 20 replicates per treatment). Penetration resistance was also determined in the laboratory with a high spatial resolution micropenetrometer (cone: 3.0 mm diameter, 30◦ semi-angle; shaft: 1.7 mm diameter and 120 mm long) (Munkholm et al., 2002). This technique was applied to the T2 field sampled soil columns at their sampling water content. The samples were placed on a x–y table keeping the seed row parallel to the y-axis. Penetrations were performed in a 10 mm × 10 mm x–y grid with a strain rate of 0.17 mm s−1 and to a maximum of 70 mm depth. Penetration resistance was recorded for Table 2 Water content at 60–100 mm depth at the time of field measurements and sampling at T1, T2 and T3 (determined by the neutron method)

PL DD-D DD-C

T1 (m3 /100 m3 )

T2 (m3 /100 m3 )

T3 (m3 /100 m3 )

15 22 23

15 17 20

24 23 25

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every 0.5 mm increment. The 200 mm high soil column was analysed in three sequences of measurements with the penetrometer. First penetration was carried out from the surface to 60 mm depth. Then the top 55 mm was excavated and penetration continued to 115 mm depth. Again the top 55 mm of the soil column was excavated and penetration continued down to 150 mm depth. From the excavated soil a sub-sample was taken and dried in the oven at 105 ◦ C for 24 h to determine the gravimetric water content.

peaks and high values. To reduce the effect of stones, median values were calculated at all depths for each position from the seeding row. These data were used for drawing contour-diagrams based on linear interpolation and to calculate the frequency of data-points exceeding a critical high penetration resistance. Averages for these frequencies were calculated for each block and used in F-tests, taking the treatment×block interaction as the residual error.

2.5. CT-scanning

3. Results

A Siemens Somatom Plus S medical CT-scanner was used to scan soil columns taken at T2 across the seeding row with 18 consecutive slices with thickness of 10 mm. The pixel size was 0.58 mm. The X-ray beam was set to 120 kV.

3.1. Field tests on density and penetration resistance

2.6. Data analysis Field measurements. The field penetration resistance data were best fitted by a log-normal distribution and transformed to yield normality. The bulk density field data were best fitted by a normal distribution. Averages were calculated for each plot and used in the calculation of mean and standard error. Micropenetration. The tested soil was rather rich of stones, which resulted in both missing data (penetration stopped due to overload) and stone induced

In the first year after converting to direct drilling— although 19 months after the last time of ploughing— significantly higher soil strength was found in direct drilled than in ploughed soil. Below 40 mm depth both direct drilling techniques yielded about three times higher penetration resistance than did PL (i.e., maximum approximately 1.2 and 0.4 MPa, respectively) (Fig. 2, T1). There was also a tendency to higher bulk density for the direct drilled soil immediately below the seeding depth (i.e., 50–80 mm depth) although significant only at 70 mm depth (Fig. 3, T1). Stronger difference between direct drilled and PL developed from T1 to T2 and stabilised at T3 for both penetration resistance and bulk density

Fig. 2. Penetration resistance determined in the field at 0–150 mm depth after one, two and three times of direct drilling (i.e., T1, T2 and T3, respectively). PL: conventional ploughed soil; DD-D: direct drilled with single-disc coulters; DD-C: direct drilled with chisel coulters. Bars indicate ±1 standard error.

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Fig. 3. Dry bulk density determined from wet bulk densities measured with a dual probe gamma-ray transmission technique and volumetric water content measured by a neutron method. Measurements performed at 0–100 mm depth after one, two and three times of direct drilling (i.e., T1, T2 and T3, respectively). PL: conventional tillage, DD-D: direct drilled with single disc coulters, DD-C: direct drilled with chisel coulters. Bars indicate ±1 standard error.

(Figs. 2 and 3). Below seeding depth (50–100 mm), bulk density increased from ca. 1.46 to 1.55 g cm−3 from T1 to T2 in the direct drilled soils. Also penetration resistance increased considerably from T1 to T2 (i.e., from ca. 1.2 to 2.2 MPa) and thereby exceeding the critical limit for root growth at 2 MPa proposed by Bennie (1996). The small differences in water content at the time of measurements (Table 2) may have influenced soil strength. However, the strong increase in penetration resistance for the direct drilled soils is undoubtedly primarily related to the increase in bulk density. The field penetration resistance displays a peak just below seeding depth (i.e., 40–60 mm) in the direct drilled soil (Fig. 2) suggesting a direct compaction effect of the drills employed. Peaks in penetration resistance were found closer to the soil surface at T1 than at T2 and T3. Pilot studies showed that this could be related to the difference in the penetration cone used (i.e., 30◦ and 60◦ semi-angle in T1 and T2/T3, respectively) (data not shown). That is, the 30◦ semi-angle cone integrated over a larger depth interval than the 60◦ semi-angle cone. At T1 and T3 the loosening action of the chisel coulter gave significantly looser soil above seeding depth than for the single-disc coulter as indicated by the penetration resistance and bulk density data. Surprisingly, the field data revealed no clear difference at T2.

3.2. Laboratory test on penetration resistance One column was selected from each treatment and used for construction of contour-diagrams (Fig. 4). The diagrams are based on median values at different combinations of depth and lateral positions from the seed row. In the PL soil a weak 40–50 mm layer (penetration resistance: <0.5 MPa) overlies a stronger lower part of the plough layer. In the direct drilled soil a weak 30 mm layer (penetration resistance: <0.5 MPa) overlies a strong 50–100 mm layer (penetration resistance: 1.5–3 MPa). At >100 mm depths of direct drilled soil, the penetration resistance decreases although still potentially root growth limiting (i.e., a high frequency of penetration resistance >2.0 MPa). The seed furrow is hardly observable for the DD-D treatment. Apparently the press wheel has successfully closed the furrow opened by the single-disc coulter. In contrast, an approximately 40 mm broad and 50 mm deep furrow is observable for the DD-C treatment. 3.3. CT-scanning The CT-scanning images shown in this paper were selected among a larger number to be representative for the different treatments (Fig. 5). For the treatments with direct drilling, the cores shown are identical to those presented in Fig. 4. For the PL treatment the data shown in Figs. 4 and 5 are from different cores.

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Fig. 4. Contour-diagrams based on penetration resistance determined at different positions from the seed row at 0 to ca. 150 mm depth with a micropenetrometer on undisturbed soil columns (180 mm diameter, 200 mm height) taken at T2. Each diagram is based on measurements from a single sample. PL: conventional ploughed soil (water content: 12.3 g/100 g), DD-D: direct drilled with single disc coulters (water content: 9.2 g/100 g), DD-C: direct drilled with chisel coulters (water content: 10.4 g/100 g).

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Fig. 5. CT-scanning images showing variations in bulk density, with the seed furrow at the centre. Bulk density increase with brightness. Each image is from a single undisturbed column (180 mm diameter, 200 mm height) from each treatment taken at T2. PL: conventional ploughed soil; DD-D: direct drilled with single-disc coulters; DD-C: direct drilled with chisel coulters.

The images clearly confirm the marked change in topsoil structure after 2 years of direct drilling. The PL soil has a disturbed and mixed structure with some dense blocky clods (bright areas) embedded in a generally porous granular soil (dark areas). This reflects the fragmenting and inverting action of the mouldboard plough. The direct drilled soils have a dense and coarse platy structure below a shallow (<30 mm) porous surface layer. The zone with platy structure extends to approximately 80 mm depth for the DD-C soil, whereas it extends to approximately 180 mm depth for the DD-D soil. A platy structure produced by direct drilling has been reported earlier by, e.g., Drees et al. (1994) and VandenBygaart et al. (1999) for near surface silt loam soils (top 50 mm). A few vertical biopores are observable for the DD-D treatment. The development of large channel-like pores after converting to direct drilling is in accordance with many other studies (e.g., Schjønning, 1989; Tebrügge and Düring, 1999). The images confirm that the DD-D seeding disturbed the surface soil less than the DD-C treatment. The seed furrow in the DD-D treatment is hardly discernible, which is consistent with the micropenetration diagrams. In contrast, the DD-C seed furrow is very clear and approximates an inverted T to rectangular shape. This inverted T-shape would be in accordance with findings of Dixon (1972, cited in Baker (1976)) for another chisel coulter. Interestingly, the lateral cracks have an open U shape in the DD-D soil in comparison with the straight lateral cracks found in the DD-C soil.

4. Discussion 4.1. Temporal effect of direct drilling Irrespective of coulter type, direct drilling caused compaction of the lower part of the arable layer on the sandy loam soil. That is, a higher bulk density and a considerable increase in penetration resistance was found below seeding depth (>40 mm). Direct drilling created already in the first year a substantial compaction in the lower part of the arable layer. Further compaction occurred in the second year (T2) and the differences stabilised after three times of treatment (T3). The immediate and substantial change in topsoil structure after adapting reduced tillage is in accordance with other results from temperate soils (Voorhees and Lindstrom, 1984; Carter, 1987, 1991). The compaction of the lower part of the arable layer in the direct drilled soil at T1 (i.e., 19 months after last ploughing) in this study may be ascribed to natural soil consolidation and compaction induced by the direct drills, as sampling and field testing was performed between wheel tracks. The results confirm that medium textured sandy loams are very prone to compaction. The presented results yield further evidence to the statement that soil compaction is a serious concern on sandy loams when introducing direct drilling in a moist cool climate (Ehlers and Claupein, 1994). Still, investigations in direct drilling on these soils are of strong interest in Denmark where they contribute a large part of the arable soils.

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The results reported in this paper are from the early stage in the conversion to direct drilling. The question is whether a combination of cautious field traffic and biological activity may counterbalance the compaction related to natural soil consolidation and field traffic in the long run. Voorhees and Lindstrom (1984) showed a recovery within 3–4 years of porosity in the 0–150 mm layer for a silty clay loam after conversion to non-inversion tillage (i.e., not direct drilling). A full recovery is not expected for the sandy loam soil as it was converted to direct drilling and display low shrinking/swelling potential, i.e., low self-mulching ability. Over time the development of a system of vertical biopores produced by earthworms and roots may facilitate deep rooting and thereby counteract the effect of early stage compaction. However, it takes time for an earthworm population to develop. VandenBygaart et al. (1999) found that it takes more than 4 years with direct drilling to develop the same number of 0.1–1.0 mm pores as in conventional tillage for a silt loam Canadian soil. Visual observations of earthworm activity was carried out in a parallel experiment on the same location as in this study (Jensen, 2002). A larger number of earthworm channels were found in the direct drilled treatments (1–5 dm−2 ) than in the ploughed soil (<1 dm−2 ) 3 years after converting to direct drilling. 4.2. Coulter type The results suggest that the chisel direct drill provided a more favourable soil environment for seed germination and seedling growth than the single disc drill. The DD-C treatment yielded an approximately 40 mm surface layer of loose granular soil above seeding depth, whereas the DD-D showed very little effect on surface soil structure. The loosening of soil above seed depth may markedly improve soil conditions for seed germination and seedling growth under wet conditions. The loosened zone may improve aeration, produce higher topsoil temperatures, and improve soil/seed contact. Other investigations have shown that some kind of loosening of surface soil may be needed to create an optimal seedbed especially for spring grown crops in moist and cool climates (Vyn and Raimbault, 1993; Vyn et al., 1998; Rasmussen, 1999). Both direct drilling techniques produced potentially root-restricting compaction in the lower part

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of the arable layer. However, field as well as laboratory results indicate that the DD-D single-disc drill—contrary to the DD-C chisel drill—in itself contributed to the compaction of the soil. The field data indicate higher penetration resistance at the base of the seed furrow (40–60 mm depth) in the DD-D than in the DD-C treatment at T1 and T3. The laboratory tests with the micropenetrometer indicate that the compaction effect of DD-D may be more important than suggested by the field tests. More than 50% of the median values for penetration resistance recorded at 40 to ca. 150 mm depth exceeded 2 MPa for the DD-D treatment in comparison with 25 and 7% for DD-C and PL, respectively (Table 3). Using 1.5 MPa as the critical limit resulted in 79, 64 and 30% of the median values with critical high penetration resistance for DD-D, DD-C and PL, respectively. The laboratory penetrometer tests revealed no clear difference in the compaction effect for different distances from the direct drilling seed furrows. This was not surprising, as, at T2, a large part of the area has been directly affected by the direct drilling (i.e., the seed row at T2 was established in the inter-row at T1). Also the CT-scanning images indicate a direct compaction effect of the DD-D direct drilling coulter. That is, the platy structure with broad U-shaped lateral cracks centred around the seed furrow found at 30 to ca. 180 mm depth for the DD-D treatment may be regarded as a result of soil compaction from the single-disc coulters. In comparison, the platy structure extended only to about 80 mm depth for the DD-C Table 3 Percent of median penetration resistance values exceeding critical high levelsa Median penetration resistance (%) >1.5 MPa

PL DD-D DD-C

>2.0 MPa

Mean

S.E.b

Mean

S.E.

30 ac 79 c 64 b

13 8 9

7a 52 b 24 ab

3 10 7

a Based on all the micropenetration measurements recorded within the 40 to ca. 150 mm depth. Average water content at measurement: 13, 11 and 13 g/100 g for PL, DD-D and DD-C, respectively. b Standard error (n = 4). c Figures followed by the same lettering with each column are not significantly different at the P = 0.05 level.

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treatment and the lateral cracks were almost horizontal. The direct compaction effect of disc drilling shown in this study is in accordance with other studies (Wilkins et al., 1983; Tessier et al., 1991). 4.3. Perspectives for direct drilling on sandy loam Despite the better performance of the chisel direct drill coulter our results confirm that soil compaction is a critical concern when adapting direct drilling on weakly structured sandy loam soils in a moist and cool climate. This means that periodical loosening of the lower part of the arable layer may be needed. Non-inversion loosening should be preferred in order to minimise the mixing of soil layers and thereby destroying the stable surface structure normally developing under direct drilling. The design of direct drills with zone loosening at adjustable depth (50–200 mm) is among future challenges on humid sandy loams. Future research is also needed to clarify whether a combination of cautious traffic and stimulated biological activity in the long-term can alleviate the effect of initial soil compaction produced by direct drilling on weakly structured soils.

5. Conclusions We conclude that irrespective of coulter type, direct drilling gave a fast compaction of the arable layer below seeding depth on the tested sandy loam. Soil strength was substantially higher already in the first year of direct drilling. Critical high penetration resistance and bulk density levels were reached after two times of treatment and stayed at the same level after three times of treatment. The chisel direct drill coulters produced a more favourable soil environment for crop establishment than the single-disc drill coulters. The chisel coulters produced a surface layer approximately 40 mm loose granular soil above seeding depth and produced less compaction at the base of the seed furrow. We conclude that soil compaction is still a critical concern when adapting direct drilling on sandy loam in a moist and cool climate. Evidence suggest that periodic non-inversion soil loosening of the lower part of the arable layer tillage is needed irrespective of the applied direct drilling technique.

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